A flexible energy-carrying circuit device may involve use of a pressure-sensitive adhesive to adhere energy-carrying filaments of the device to a substrate. For example, FIG. 1 shows a fragmented cross-sectional view of a typical flexible optical circuit apparatus in which an adhesive is used to secure the fibers to a substrate. The apparatus 10 comprises a plurality of optical fibers 12, encapsulated in an encapsulating sheet 14, such as polyurethane, nylon, polypropylene, KAPTON,.TM. doped MYLAR,.TM. or aluminum foil. The encapsulating sheet 14 protects the optical fibers, provides structural stability, and keeps the fibers in place during handling, yet it also should be flexible so that the apparatus may be mounted in an appropriate structure or connected to other systems or machines. A pressure-sensitive adhesive 16 is used to adhere the encapsulated fibers to a substrate 18. The substrate 18 is typically comprised of a flexible plastic and advantageously is fabricated with a polymer of the type commercially identified as KAPTON.TM., which is non-flammable and does not melt. A material that previously has been used for the pressure-sensitive adhesive 16 comprises number 711 adhesive, a resin available from Adchem Corporation of Westbury, N.Y. For further background regarding fiber optic structures, encapsulation techniques, and materials and methods used in these structures, see U.S. Pat. No. 5,582,673 to Burack and Ling et al (inventors herein), issued Dec. 10, 1996, entitled "Optical Fiber Encapsulation Techniques," and U.S. Pat. No. 5,259,051 to Burack et al. (an inventor herein), issued Nov. 2, 1993, entitled "Optical Fiber Interconnection Apparatus and Methods of Making Interconnections," both of which are hereby incorporated by reference.
A drawback with this structure, however, is that the encapsulating sheet 14 particularly when comprised of thermoplastic material may melt when heated, affecting the structural integrity of the apparatus. More importantly, when the thermoplastic melts, for example, in response to a flame, it may expose the adhesive 16 to the air, causing the adhesive when flammable to ignite. Also, the temperature of these devices as well as other electronic devices employing adhesives may increase during operation, leading to flammability concerns.
It is desirable that optical circuits and other energy-carrying devices meet certain levels of flame retardancy. Optical circuits and electronic devices are tested for flame retardancy pursuant to standards known in the industry for measuring the flammability of plastics used in electronic devices and appliances, namely, the Underwriters' Laboratory (UL) 94 standards. The UL standards are well known and are also described in M. Robert Christy, Standards, Bans, and Flame Retardants, PLASTICS COMPOUNDING (September/October 1993), at pp. 59-61. The UL 94 vertical (UL94V) standards have been applied to optical circuit devices, including the UL94V test and the 94VTM test, with the latter test (94VTM), applicable for thinner materials prone to distortion.
The flammability of optical circuits and energy-carrying devices has been addressed in many ways, including reconfiguring the devices and replacing the thermoplastic materials with materials having higher melting points. For example, one method has been to apply a second layer of KAPTON.TM. or other non-flammable coversheet 20 (FIG. 1A), over the thermoplastic encapsulant, which protects the thermoplastic and reduces the likelihood that it will melt. Another method involves rolling the system into a cylinder with the KAPTON.TM. substrate 18 facing out (FIG. 1B). U.S. Pat. No. 5,582,673, referenced above, describes encapsulation techniques involving materials other than thermoplastic in addressing flammability issues. While these methods are effective in reducing the flammability of the circuit overall, they limit the flexibility of the device and the configurations in which, and materials with which, the device can be fabricated. It would be beneficial to have a method of adhering the optical fibers to the substrate which avoids the use of flammable adhesives or involves adhesives having reduced flammability.
Developing pressure-sensitive adhesives that have good adhesive properties and yet are non-flammable and suitable for use in fiber optic and other energy-carrying devices has presented many challenges. A pressure-sensitive adhesive may be defined as a material that bonds surfaces at room temperature and with the application of some (and preferably a low) pressure. Typically, materials with good adhesive, cohesive, and tack qualities are also flammable. Pressure-sensitive materials based on acrylates or polyacrylates, for example, are tough, resilient, and flexible materials that have excellent pressure-sensitive adhesive properties, but they are also flammable.
Typically, to reduce the flammability of a pressure-sensitive adhesive, combustion-inhibiting compounds have been added to the adhesive. The most commonly-used additive is antimony trioxide which often is used in combination with halides, such as titanium tetrachloride. Making an adhesive flame retardant can be more complicated, however, than simply adding the combustion-inhibiting material, because the additive may disrupt the sensitive balance of properties of the material. For example, certain phosphates, while combustion-inhibiting, will greatly weaken the cohesive properties of the adhesives and cannot effectively be used.
The use of combustion-inhibiting additives has been found to be impractical for optical circuits. For the circuits to meet desired levels of flame retardancy as previously discussed, quantities of combustion-inhibiting additives at greater than twenty-five percent of the total solids would have to be added to the adhesive. However, typical flame retardant systems such as those based upon antimony oxide tend to settle out of acrylic coatings and adhesives, and they opacify the polymer and detract from its adhesive properties. Thus, the addition of sufficient quantities of combustion-inhibiting additives to meet flame-retardancy standards decreases the tack of the adhesives to the point that they can no longer meet desired fiber placement tolerances.
Thus, there is a need for a method of adhering energy-carrying filaments and particularly optical fibers to a substrate which avoids the use of flammable adhesives or which reduces the flammability of the adhesives while maintaining the adhesive qualities to a degree sufficient for such applications. This invention addresses this need. Further advantages may appear more fully upon consideration of the description below.